Frictional pairings for service brakes typically are selected such that the brake pads cooperating with the disk or drum are softer than the material of the disk or drum, since worn brake pads can be changed more easily and more cost-effectively than the disks or drums. For simplicity, the term “brake pads” is meant to include all brake pads having at least a lining of the specified material on the friction surface.
Upon selection of identical materials for the frictional pairing of brake pad linings and brake disks the brake disks would wear to the same extent as the brake pads; this is not desired for service brakes for the reasons set forth above.
Typically, therefore, for brake pads in service brakes, organically bound materials (using resins) are used which comprise fibers and particulate hard materials as friction materials.
Friction linings for brake pads have been known from DE-A 197 11 830, which brake pads can be used in connection with brake bodies made from fiber-reinforced ceramics and are made by sintering of mixtures of carbon and metal particles, which likewise contain, additionally, hard material particles and/or a solid lubricant. These brake pads indeed show reduced wear in combination with carbon-ceramic brake disks, compared with common organically bound linings; however, the static friction appears to still need improvement. From DE-C 197 27 586, a brake system including a friction pairing has been known wherein both brake disk and brake pads are made from a fiber-reinforced ceramic composite, and where a C/C body (a body of porous carbon reinforced with carbon fibres) is used having a density which is, at least in the region of the surface, higher than that of the C/C body used to make the brake disk. During siliconisation, less silicon can penetrate into the dense body and react with the carbon therein, which causes the body for the brake pads to comprise less silicon carbide than the body for the brake disk, and therefore, the brake pads are less hard than the brake disk. The fibre orientation in both bodies is essentially isotropic.
In contrast to brake pads for brakes used in the driving mode (service brakes, destined to decelerate a vehicle in motion to a lower velocity) which must have a sufficient resistance to abrasion and a high sliding, or dynamic, friction coefficient, only static friction counts for brake pads for parking brakes. In particular, abrasion is not a significant issue in a parking brake as there usually is no wear by dynamic friction.
Upon parking of a motor vehicle with brake disks heated by operation, of course, cooling through common ventilated structures of the brake disks does not occur. Brake disks and the brake pads of the parking brake, then, are in close contact by contact pressure of the parking brake and cool only slowly. It has been shown that with such operating conditions, the brake pads known from the state of the art in connection with carbon-ceramic brake disks are markedly affected and have too little static friction at high temperature. This insufficient static friction is already apparent in a cold operating state. As hot bodies shrink upon cooling, pressure exerted on the brake pads will also decrease upon cooling and will be insufficient to hold back a vehicle on a slope. The brake pads known from DE 197 27 586 also have the disadvantage that their hardness and temperature resistance due to their lower density in the siliconised state, and their heat conductivity are insufficient for use in a parking brake.
A further disadvantage of the brake pads known from the state of the art for parking brakes is the fact that an interlayer between the brake disk and the brake pad of the parking brake is formed after longer standing time of a motor vehicle, in particular in moist surroundings, by corrosion on the contact surface between the brake pad and brake disk; this condition is referred as “the brake is corrosion locked”. Upon loosening of this connection (requiring a garage stop), residues of this interlayer remain on the brake disk which then must be trimmed off or replaced.
Therefore, brake pads for parking brakes are needed which are not damaged with high temperatures in contact with carbon-ceramic brake disks, and which also provide the highest possible static friction in a cold operating state in combination with carbon-ceramic brake disks. The motor vehicle in question must be held reliably also at inclinations of 45° and more; this is not possible with linings or brake pads known to this point in time as described supra. Further problems are avoiding corrosion locking of the brake pads on the brake disk and providing a high heat conductivity within the brake pads.